Question

Rez. ec:
cos(x)+cos(2x)+cos(3x)+...cos(nx)=n.

Answer 1

cosx=1 are ca solutie x=2kπ, pentru ∀k∈Z, deci si  cos[p(2kπ)]=1, unde p∈Z, rezulta solutia x∈{2kπ| k∈Z}